When I converted my 18th scales, I was getting 10-12min on gp1100s (yeah, it was a while ago, lol) and run time jumped to 20-22min with the same mah lipo. Now lipos have gotten better, I can put a lot more mah in the same space with lipo and run times are 30-35min.

ALl the heat a NiMh makes when its being discharged is energy wasted and prob most of teh reason lipo gives better run times. A proper setup shouldn't be making lipo anything more than just warm when its run.